The present disclosure generally relates to a semiconductor structure, and particularly to a field effect transistor employing a gate spacer having an anisotropic dielectric constant and a method of manufacturing the same.
A gate spacer on a field effect transistor has conflicting requirements in terms of a dielectric constant. On one hand, a strong coupling is desirable between a gate electrode and each of a source extension region and a drain extension region. To increase coupling with the source and drain extension regions (or portions of the source region and the drain region that underlap the gate spacer), it is desirable to have a high dielectric constant for a gate spacer. On the other hand, a weak coupling is desirable between the gate electrode and each of source/drain contact via structures and portions of the source and drain regions that do not underlap with the gate spacer. To reduce coupling with the source/drain contact via structures and portions of the source and drain regions that do not underlap with the gate electrode, it is desirable to have a low dielectric constant for the gate spacer. As known in the art, a single dielectric material having an isotropic dielectric constant is selected for the gate spacer such that the dielectric constant of the dielectric material is an optimized value. Typically, silicon nitride having a dielectric constant of 7.9 is selected as the dielectric material of the gate spacer.